1. Field of the Invention
This invention relates to a device that limits the longitudinal loads on an electrical transmission structure and particularly to a device to limit the longitudinal loads on an electrical transmission structure, which includes an energy dissipating mechanism that safely dissipates the energy released when a failure releases the tension in the wires.
2. Description of the Prior Art
On electrical transmission lines, longitudinal loads act parallel to the wires that carry electricity. Transverse loads act horizontally perpendicular to the wires and vertical loads act downward or upward. On the majority of transmission towers, the wires are suspended from the structure as shown in FIG. 1. Under normal loading, a transmission structure with suspended wires is loaded principally in the vertical and transverse directions. The longitudinal loads are typically very small when the wires are suspended. As long as these loads do not exceed the design limits of a tower or other structure, the tower is safe. Circumstances can arise, however, in which these load limits can be exceeded.
An example of this occurs when the wires on one side of the tower are heavily loaded with ice and the wires on the opposite side are bare. Another example of this is when one of the conductors or shield wires breaks. These events cause large unbalanced longitudinal loads on the tower that can cause tower failure. One longitudinal tower failure can transfer load to the adjacent tower causing it to also fail leading to a cascading failure of many towers in a particular line.
To prevent collapse of the line, devices are used to limit the force on the tower due to unbalanced ice loads and loads that occur when a line break or similar failure occurs. Two approaches to limiting the damage are represented in the prior art. The first is to install a mechanical fuse, which releases the wire from the tower at a predetermined force. The second is to install an energy-absorbing element that remains connected to the tower.
Examples of mechanical fuses are found in the following U.S. patents: U.S. Pat. No. 3,150,229 teaches a link type device that is designed to release under the proper longitudinal force. This causes the line to drop away from the tower, thereby protecting the tower from excessive forces. U.S. Pat. No. 3,519,727 teaches a device for releasing a “V” string insulator line during a failure. This is designed to reduce the pulling load on the tower in such a line failure and includes anti-friction pads. U.S. Pat. No. 3,711,049 teaches a load limiter device that is designed to drop a line from a tower under excessive force. By dropping the line, the tower can be saved.
The problem with these mechanical fuses is that they all have limitations on their use. They are complicated devices with many parts. Freezing rain and fog can cause buildups of ice that prevent their proper operation. Anti-friction devices are needed for consistent operation when the weight of the wires changes due to ice buildup.
Examples of energy absorbing devices incorporated in the insulator assemblies of the tower are found in the following U.S. Pat. No. 4,791,243 teaches an energy absorption device that has a non-spring helix that is placed between a tower and the insulator string. The device deforms by plastically uncoiling in response to excessive forces. U.S. Pat. No. 5,898,558 teaches a protective overload device that uses a cable embedded in a frangible matrix to absorb energy during a failure. After the device has operated, the cable and frangible matrix can be replaced without having to replace the entire unit.
The problem with these units is that they operate on the combined vertical, transverse and longitudinal loads. A balanced heavy ice load can trigger unintended operation of the unit that removes the protection against broken wires.
U.S. Pat. No. 3,005,866 teaches a connector that attaches the line to the insulator. The connector has a pair of coils attached between two parts, one of which is connected to the insulator and the other to the conductor. When a line breaks, the connector rotates in relation to the insulator, allowing the two parts to separate. The coils then extend, thereby reducing the shock on the tower and possible preventing the line from dropping to the ground. The problem with this device is that the insulator is typically made up of flexible units that are not stiff enough to ensure the rotation of the connector in relation to the insulator to allow activation of the coils.
Finally, U.S. Pat. No. 3,211,260 teaches an arrester that has a large body of material held in a housing. When excessive force is applied, the material is pulled out of the housing, over a series of pins. This action absorbs the energy as the material is pulled from the housing.